1. Field of the Invention:
The present invention relates to a solid state imaging device which can reduce smear and a manufacturing method thereof.
2. Description of the Prior Art:
As CCD (charge coupled device) solid state imaging device for use in CCD cameras or the like, there are generally known a CCD solid state imaging device (horizontal overflow CCD) which discharges saturated electrical charges in the horizontal direction and a CCD solid state imaging device (vertical overflow CCD) which discharges saturated electrical charge in the substrate direction, i.e., vertical direction.
The former CCD, i.e., the horizontal overflow CCD has a problem that electrical charges, photoelectrically-converted in the region except a depletion layer in the light sensing area enter a vertical register to cause a false signal (smear). Whereas, in the latter CCD, i.e., the vertical overflow CCD, electrical charges, photoelectrically-converted, in the region except the depletion layer of the light sensing area can be discharged toward the substrate side and therefore generates less smear as compared with the horizontal overflow CCD.
FIG. 1 of the accompanying drawings shows a structure of a solid state imaging device based on the conventional vertical overflow CCD. As shown in FIG. 1, an N-type light sensing region 3, a vertical register 4 and a P-type channel stopper region 5 are formed within a first P-type well region 2 on an N-type silicon substrate 1. A P-type positive electrical charge storage region 6 is formed on the surface of the light sensing region 3 and a second P-type well region 7 is formed beneath the vertical register 4, respectively. A transfer electrode 9 formed of a polycrystalline silicon layer is selectively formed on the vertical register 4 through a gate insulating layer 8. An Al light-intercepting layer 11 is formed on the transfer electrode 9 through an interlevel insulator 10, and a surface protecting layer 12 formed of an plasma SiN layer, for example, is formed on the whole surface including the Al light-intercepting layer 11. A P-type region formed between the light sensing region 3 and the vertical register 4 forms a read-out gate 13.
The Al light-intercepting layer 11 is selectively removed on the light sensing region 3 by the etching process and a light L is introduced into the light sensing region 3 through a hole 14 formed by this etching process.
In the conventional CCD solid state imaging device, however, the Al light-intercepting layer 11 is selectively removed on the light sensing region 3 by the etching process after the Al light-intercepting layer 11 was formed on the interlevel insulator 10 so that a transparent film such as the gate insulating layer 8, the interlevel insulator 10 or the like is continuously formed from the lower portion of the hole 14 of the Al light-intercepting layer 11 up to the upper portion of the vertical register 4. As a result, of the light L that became incident on the light sensing region 3 side, a diffracted light thereof is introduced through the interlevel insulator 10 and the gate insulating layer 8 to the vertical register 4 side and a part of the light component thus introduced becomes incident on the vertical register 4 to thereby cause a smear to occur.
According to the prior art, in order to reduce the smear, it is proposed to reduce a thickness of the interlevel insulator 10 formed between the Al light-intercepting layer 11 and the light sensing region 3. There is already a limit that the smear is reduced by forming the interlevel insulator 10 as a thin film.
Further, an amount in which a smear occurs after the surface protecting layer 12 was formed is increased about twice as compared with that of the case where the surface protecting layer 12 is not formed, resulting in the picture quality being deteriorated. The reason for this is that, as shown by a one-dot chain line frame D in FIG. 1, since the upper portion on the edge portion of the Al light-intercepting layer 11 which determines the hole 14 of the light sensing region 3 is formed as an acute angle portion, the shape of the surface protecting layer 12 formed thereon becomes a spherical shape having a small curvature and becomes a convex lens portion 12s.
More specifically, the light L incident from above is considerably refracted at the aforesaid convex lens portion 12s and therefore advanced toward the vertical register 4. As a result, in addition to an oblique light component L.sub.1 that is obliquely introduced into the vertical register 4, a reflected light component L.sub.2 introduced into the vertical register 4 while being reflected between the silicon surface and the under surface of the Al light-intercepting layer 11 is increased, which as a result causes a smear to occur frequently.
The cause that the shape of the surface protecting layer 12 at its portion formed on the peripheral edge of the light sensing region 3 becomes the convex lens (12s) is that, as earlier noted, the coverage of the surface protecting layer 12 is deteriorated because the edge shape of the Al light-intercepting layer 11 is the acute angle. However, since the hole 14 on the light sensing region 3 must be formed with high accuracy and stability, the Al light-intercepting layer 11 must be subjected to the patterning by the anisotropic-etching such as the RIE (reactive ion etching) or the like. There is then the disadvantage such that the edge shape of the Al light-intercepting layer 11 becomes acute unavoidably.